Method and apparatus for detecting motion between odd and even video fields

ABSTRACT

A method for measuring motion at a horizontal and vertical position between video fields of opposite parity comprising the steps of measuring the signal values of at least two vertically adjacent pixels from a video field of one parity and at least two vertically adjacent pixels from a video field of the opposite parity such that when taken together, the pixels represent contiguous samples of an image at said horizontal and vertical position, and determining whether the signal value of any of the pixels lies between the signal values of adjacent pixels in the field of opposite parity and in response outputting a zero motion value, otherwise, outputting a motion value equal to the lowest absolute difference between any of the pixels and its closest adjacent pixel in the field of opposite parity.

FIELD OF THE INVENTION

This invention relates in general to digital video signal processing andmore particularly to a method and apparatus whereby motion between oddand even video fields may be reliably measured despite the presence ofhigh vertical spatial frequencies.

BACKGROUND OF THE INVENTION

The NTSC and PAL video standards are in widespread use throughout theworld today. Both of these standards make use of interlacing in order tomaximize the vertical refresh rate thereby reducing wide area flicker,while minimizing the bandwidth required for transmission. With aninterlaced video format, half of the lines that make up a picture aredisplayed during one vertical period (i.e. the even field), while theother half are displayed during the next vertical period (i.e. the oddfield) and are positioned halfway between the lines displayed during thefirst period. While this technique has the benefits described above, theuse of interlacing can also lead to the appearance of artifacts such asline flicker and visible line structure.

It is well known in the prior art that the appearance of an interlacedimage can be improved by converting it to non-interlaced (progressive)format and displaying it as such. Moreover, many newer displaytechnologies, for example Liquid Crystal Displays (LCDs), arenon-interlaced by nature, therefore conversion from interlaced toprogressive format is necessary before an image can be displayed at all.

Numerous methods have been proposed for converting an interlaced videosignal to progressive format. For example, linear methods have beenused, where pixels in the progressive output image are generated as alinear combination of spatially and/or temporally neighbouring pixelsfrom the interlaced input sequence.

Although this approach may produce acceptable results under certainconditions, the performance generally represents a trade off betweenvertical spatial resolution and motion artifacts. Instead of accepting acompromise, it is possible to optimize performance by employing a methodthat is capable of adapting to the type of source material. Forinstance, it is well known that conversion from interlaced toprogressive format can be accomplished with high quality for sourcesthat originate from motion picture film or from computer graphics (CG).Such sources are inherently progressive in nature, but are transmittedin interlaced format in accordance with existing video standards. Forexample, motion picture film created at 24 frames per second isconverted to interlaced video at 60 fields per second using a processknown as 3:2 pull down, where 3 fields are derived from one frame and 2are derived from the next, so as to provide the correct conversionratio. Similarly, a computer graphics sequence created at 30 frames persecond is converted to interlaced video at 60 fields per second using apull down ratio of 2:2, where 2 fields are derived from each CG frame.By recognizing that a video sequence originates from a progressivesource, it is possible for a format converter to reconstruct thesequence in progressive format exactly as it was before its conversionto interlaced format.

Unfortunately, video transmission formats do not include explicitinformation about the type of source material being carried, such aswhether the material was derived from a progressive source. Thus, inorder for a video-processing device to exploit the progressive nature offilm or CG sources, it is first necessary to determine whether thematerial originates from a progressive source. If it is determined thatthe material originates from such a source, it is furthermore necessaryto determine precisely which video fields originate from which sourceframes. Such determination can be made by measuring the motion betweensuccessive fields of an input video sequence.

It is common to measure at least two different modes of motion indetermining the presence of a film source. Firstly, it is common tomeasure the motion between a given video field and that which precededit by two fields. In this case, motion can be measured as the absolutedifference between two pixels at the same spatial position in the twofields. A measure of the total difference between the two fields can begenerated by summing the absolute differences at the pixel level overthe entire field. The quality of the motion signal developed in this waywill be fairly high, since the two fields being compared have the sameparity (both odd or both even) and therefore corresponding samples fromeach field have the same position within the image. Thus any differencethat is measured between two pixels will largely be the result ofmotion. Although the quality of measurement made in this way is high,unfortunately it is of limited value. For an input sequence derived fromfilm in accordance with a 3:2 pull down ratio, only one out of fivesuccessive measurements made in this way will differ significantly fromthe rest. The measure of motion between the first and third fields ofthe three fields that are derived from the same motion picture framewill be substantially lower than the measurements obtained during theother four fields, since the two fields being compared are essentiallythe same and differ only in their noise content. This does not providesufficient information to avoid artifacts under certain conditions whena film sequence is interrupted. Also, in the case of an input sequencederived from film or CG in accordance with a 2:2 pull down ratio, nouseful information is provided whatsoever.

A second mode of motion that can be measured is the motion betweensuccessive fields which are of opposite parity (one odd and one even).Although this mode of measurement overcomes the limitations of theabove, it is inherently a more difficult measurement to make since aspatial offset exists between fields that are of opposite parity. Thus,even if there is no actual motion, a finite difference between thefields may exist owing to the spatial offset. This tends to increase themeasured difference when there is no motion making it more difficult toreliably discriminate between when there is motion and when there isnot. This is particularly true in the presence of noise and/or limitedmotion. A number of methods have been proposed in the prior art for themeasurement of motion between fields of opposite parity. It is anobjective of the present invention to provide a method for themeasurement of motion between fields of opposite parity with greaterability to discriminate between the presence of motion or lack thereofthan those of the prior art.

Various techniques besides those linear methods described above, havealso been proposed for conversion from interlaced to progressive formatof video material not derived from film. For example, if it can bedetermined whether specific parts of an image are in motion, then eachpart can be processed accordingly to achieve more optimal results. Thisrequires the measurement of motion locally and is akin to the problem ofmeasuring motion globally as required to determine the presence of filmsources. The same elemental operations may be used to measuredifferences at a pixel level, only in the latter case the differencesare summed over an entire field to produce a global measurement, whereasin the former case the difference may be used as a measure of localmotion without further summation. As with the global case, the localcase may involve various modes of measurement. One of the modes that canbe used to advantage is the local measurement of motion betweensuccessive fields of opposite parity. It is a further objective of thepresent invention to provide such a method.

The following patents are relevant as prior art relative to the presentinvention:

U.S. Pat. Documents 5,689,301 - Nov. 18, 1997 Method and apparatus foridentifying Christopher video fields produced by film sources6,014,182 - Jan. 11, 2000 Film source video detection Swartz 4,932,280 -Jan. 1, 1991 Motion sequence pattern detector for Lyon video 5,291,280 -Mar. 1, 1994 Motion detection between even and odd Faroudja fieldswithin 2:1 interlaced television standard

SUMMARY OF THE INVENTION

According to the present invention, a method and apparatus are providedwhereby the motion between two fields of opposite parity may be measuredwith greater ability to discriminate between the presence of motion andlack thereof than with those techniques of the prior art. According tothe present invention, the level of motion between the two fields at aspecific position is determined by comparing the values of fourvertically adjacent pixels, each of which having the same horizontalposition, where the first and third pixels are taken from verticallyadjacent lines in one field, the second and fourth pixels are taken fromvertically adjacent lines in the other field such that the verticalposition of the second pixel is halfway between the first and thirdpixels and the vertical position of the third pixel is halfway betweenthe second and fourth pixels. If the value of the second pixel liesbetween the values of the first and third pixels, of if the value of thethird pixel lies between the values of the second and fourth pixels,then the local motion is taken as zero. Otherwise, the local motion istaken as the minimum of the absolute differences between the first andsecond pixels, the second and third pixels, and between the third andfourth pixels.

This technique has the benefit that false detection of motion arisingfrom the presence of high vertical spatial frequencies is minimized,while actual motion is still readily detected. Using this technique,false detection is completely avoided for vertical spatial frequenciesless than one half of the vertical frame Nyquist frequency. Utilizingmore than four pixels extends the range of vertical spatial frequenciesfor which false detection is completely avoided irrespective of thevertical frame Nyquist frequency. In general, if the method of thepresent invention is scaled to utilize n pixels where n is greater thanor equal to four, then false detection of motion is avoided forfrequencies up to and including (n−3)/(n−2) of the vertical frameNyquist frequency. In any case, the resulting local measurement ofmotion can either be used directly or summed over an entire field inorder to provide a global motion signal that is useful for determiningwhether an input sequence derives from a film source.

According to a further aspect of the present invention, the contributingpixels are chosen such that their spatial positions remain constantregardless of whether the most recent of the two fields is even or odd.In this way, any motion that is falsely detected in a static imageremains constant from one field to the next, thereby improving theability to distinguish between falsely detected motion and actual motionthat arises as a result of a sequence that was generated in accordancewith a 2:2 pull down ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

A description of the prior art and of the preferred embodiments of thepresent invention is provided hereinbelow with reference to thefollowing drawings in which:

FIG. 1 is a schematic representation showing how motion may be measuredbetween successive fields of opposite parity, according to the priorart.

FIG. 2 is a schematic representation showing how motion may be measuredbetween successive fields of opposite parity using a second method,according to the prior art.

FIG. 3 is a schematic representation showing how motion may be measuredbetween successive fields of opposite parity using a third method,according to the prior art.

FIG. 4 is a schematic representation showing how motion may be measuredbetween successive fields of opposite parity using a fourth method,according to the prior art.

FIG. 5 is a schematic representation of a first example showing howmotion may be measured between successive fields of opposite parity,according to the method of the present invention.

FIG. 6 is a schematic representation of a second example showing howmotion may be measured between successive fields of opposite parity,according to the method of the present invention.

FIG. 7 is a schematic representation showing how motion may be measuredbetween successive fields of opposite parity, according to analternative embodiment of the method of the present invention.

FIG. 8 is a block diagram of an apparatus for implementing the method ofthe present invention.

FIG. 9 is a schematic representation of a third example to show howmotion may be measured between successive fields of opposite paritywhere the most recent of the two fields is even, according to the methodof the present invention.

FIG. 10 is a schematic representation of a fourth example to show howmotion may be measured between successive fields of opposite paritywhere the most recent of the two fields is odd, according to the methodof the present invention.

FIG. 11 is a schematic representation showing how the contributingpixels are selected to have a particular spatio-temporal relationship toone another depending on whether the most recent field is even or odd,according to a further aspect of the present invention.

FIG. 12 is a schematic representation of a further example to show howmotion may be measured between successive fields to opposite paritywhere the most recent of the two fields is odd, according to the methodof the present invention.

FIG. 13 is a block diagram of an apparatus for implementing the methodas set forth in FIGS. 9, 11 and 12, according to a preferred embodimentof the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, a first example is shown of how motion may bemeasured between successive fields of opposite parity, according to onetechnique known in the prior art. The left half of FIG. 1 shows thespatio-temporal relationship between a set of vertically and temporallyadjacent pixels at a given horizontal position. It is clearly shown inFIG. 1 that the vertical position of each pixel in the even field ishalfway between the two nearest pixels in the odd field. The right halfof FIG. 1 shows the value of each pixel relative to its verticalposition. A curved line is shown connecting the pixels and is intendedto represent an image detail, the intensity of which varies verticallywithin the image in a sinusoidal fashion with the bright and dark imageportions (i.e. signal crests and troughs) occurring in the even videofield, and intermediate intensity image portions occurring in the oddfield. The curved line is drawn continuously through the pixels of boththe odd and the even fields to represent the fact that both fields arepart of an image in which there is no motion. Two pixels, P1 and P2 arehighlighted showing their spatio-temporal relationship to one anotherand their values within the image. In this example, the image detail hasa vertical spatial frequency that is exactly equal to one half of thevertical frame Nyquist frequency and a peak amplitude equal to quantityA. The formula at the bottom of FIG. 1 shows how a local measurement ofmotion is made using a first prior art technique. The motion is simplytaken as the absolute difference between the two pixels P1 and P2, asdepicted in FIG. 1. Note that although the pixel values used in thisexample are intended to represent samples of an image in which there isno motion, application of this prior art technique will result in ameasured motion value equal to quantity A. Thus, this technique fails toreject as motion the difference between pixels P1 and P2 that arisesowing to their different vertical positions.

Referring now to FIG. 2, a somewhat enhanced measurement technique isshown as fully disclosed in U.S. Pat. No. 5,291,280 (Faroudja). The lefthalf of the figure shows the spatio-temporal relationship between pixelsin two successive video fields while the right half shows the value ofeach pixel relative to its vertical position for a particular imagedetail. The example used is that of an image detail that has a verticalspatial frequency exactly equal to one half of the vertical frameNyquist frequency. The formula for calculating the motion according tothis second method is shown at the bottom of FIG. 2. The measured motionis taken as the lesser of the absolute differences between pixels P1 andP2, and between pixels P2 and P3, as depicted in FIG. 2. As before,although the pixel values used in this example are intended to representsamples of an image in which there is no motion, application of thistechnique will result in a measured motion value equal to quantity A.Thus, as with the previous method, this technique fails to reject asmotion the difference between the pixels that arises owing to theirdifferent vertical positions.

Referring now to FIG. 3, a further enhanced measurement technique isshown, as disclosed in U.S. Pat. No. 6,014,182 (Swartz). The left halfof the figure shows the spatio-temporal relationship between pixels intwo successive video fields while the right half shows the value of eachpixel relative to its vertical position for a particular image detail.The example used is that of an image detail that has a vertical spatialfrequency exactly equal to one half of the vertical frame Nyquistfrequency. The formula for calculating the motion according to thisthird method is shown at the bottom of FIG. 3. The measured motion istaken as the lesser of the absolute differences between pixels P1 andP2, and between pixels P2 and P3, unless the absolute difference betweenpixels P1 and P3 is greater than the lesser of the absolute differencesbetween pixels P1 and P2, and between pixels P2 and P3, in which casethe motion value is taken as zero. As before, although the pixel valuesused in this example are intended to represent samples of an image inwhich there is no motion, application of this technique results in ameasured motion value equal to quantity A. Thus, as with the previousmethod, this technique fails to reject as motion the difference betweenthe pixels that arises owing to their different vertical positions.

Referring now to FIG. 4, another enhanced measurement technique isshown, as disclosed in U.S. Pat. No. 5,689,301 (Christopher). The lefthalf of the figure shows the spatio-temporal relationship between pixelsin two successive video fields while the right half shows the value ofeach pixel relative to its vertical position for a particular imagedetail. The example used is that of an image detail that has a verticalspatial frequency exactly equal to one half of the vertical frameNyquist frequency. The formula for calculating the motion according tothis fourth method is shown at the bottom of FIG. 4. The measured motionis taken as the lesser of the absolute differences between pixels P1 andP2, and between pixels P2 and P3, unless the value of pixel P2 isbetween the values of pixels P1 and P3, in which case the motion valueis taken as zero. As before, although the pixel values used in thisexample are intended to represent samples of an image in which there isno motion, application of this technique results in a measured motionvalue equal to quantity A. Thus, as with the previous method, thistechnique fails to reject as motion the difference between the pixelsthat arises owing to their different vertical positions.

Referring now to FIG. 5, an enhanced measurement technique is shownaccording to the present invention. The left half of the figure showsthe spatio-temporal relationship between pixels in two successive videofields while the right half shows the value of each pixel relative toits vertical position for a particular image detail. The example used isthat of an image detail that has a vertical spatial frequency exactlyequal to one half of the vertical frame Nyquist frequency. The formulafor calculating the motion according to one aspect of the presentinvention is shown at the bottom of FIG. 5. The measured motion is takenas the lesser of the absolute differences between pixels P1 and P2,pixels P2 and P3, and between pixels P3 and P4, unless the value ofeither pixel P2 or pixel P3 is between the values of its immediateneighbours, in which case the motion value is taken as zero. Using thistechnique, the motion value generated in the example is zero, since thevalue of pixel P3 is between that of P2 and P4. This is the desiredresult, since the pixel values in the example are intended to representsamples of an image in which there is no motion. In fact, it can beshown that by using this technique, false detection of motion iscompletely avoided for vertical spatial frequencies less than one halfof the vertical frame Nyquist frequency. Although some of the prior arttechniques may avoid false motion under certain conditions, there is novertical spatial frequency below which any of the four prior arttechniques discussed above are guaranteed to avoid all false motion, asprovided by the present invention.

Referring now to FIG. 6, another example is provided in which thepresent invention is applied to an image in which motion exists. Theleft half of the figure shows the spatio-temporal relationship betweenpixels in two successive video fields while the right half shows thevalue of each pixel relative to its vertical position within the image.In this example, a continuous line has been drawn through pixels P1 andP3 from the odd field, and a separate line has been drawn through pixelsP2 and P4 from the even field to represent the fact there is no directcorrelation between the samples from the odd field and those from theeven field. Pixel values P2 and P4 differ from pixel values P1 and P2 byquantity B. According to the method of the present invention, the motionvalue is given as quantity B which is the desired result since itcorrectly indicates the presence of motion between the fields. The useof a four-pixel aperture in the present invention may result in a lowermeasured motion value near the edges of moving objects than wouldotherwise be obtained using a two or three pixel aperture as in theprior art methods. When summed over an entire field, this may tend toproduce a slightly lower total than would otherwise be obtained.However, the present technique produces significantly lower false motionvalues for fields between which there is no motion. For typical videosources, the present technique results in a significantly higher ratiobetween the values measured where motion exists and the values measuredwhere there is none. Hence, the ability to discriminate between motionand lack thereof is enhanced.

In another aspect of the present invention, utilizing greater than fourpixels extends the range of vertical spatial frequencies for which falsedetection is avoided. Referring now to FIG. 7, an example is providedwhich is similar to that of FIG. 5 except that the method has beengeneralized to make use of n pixels. The formula for calculating themotion is shown at the bottom of the figure. The example used is that ofan image detail that has a vertical spatial frequency exactly equal toone half of the vertical frame Nyquist frequency. Application of theformula in this case yields a motion value of zero, which is the desiredresult since there is no motion between the fields. It will beunderstood from FIG. 7 that for higher frequencies as well, inparticular those frequencies up to and including (n−3)/(n−2) of thevertical frame Nyquist frequency, false detection of motion iscompletely avoided.

FIG. 8 shows an apparatus implementing the method of the presentinvention as shown in FIG. 5 where a motion value is calculated based onfour pixels. An input video signal is applied to the input of a memorycontroller 10, a line delay element 12 and a first input of adifferencing circuit 14. The pixel that is present at the video input atany given time corresponds to that designated as pixel P4 in FIG. 5. Thememory controller stores incoming video data into a DRAM array 11 andlater retrieves it so as to produce a version of the input video signalthat is delayed (e.g. by 263 lines in the case of an NTSC input). Thememory controller 10 may also concurrently retrieve other versions ofthe input video signal that are delayed by different amounts to be usedfor other purposes that are not relevant to the present invention. Thepixel that is output from the memory controller 10 at any given timecorresponds to that designated as pixel P3 in FIG. 5, which issubsequently applied to the input of a second line delay element 13, afirst input of a differencing circuit 15 and the second input ofdifferencing circuit 14 referred to herein above. Line delay element 12provides a version of the input video signal that is delayed by onevertical line, and corresponds to pixel P2 in FIG. 5. Pixel P2 isapplied to a first input of a differencing circuit 16 and the secondinput of differencing circuit 15 described earlier. Line delay element13 provides a version of the delayed video signal from the memorycontroller that is further delayed by one vertical line and correspondsto pixel P1 in FIG. 5. Pixel P1 is applied to the second input ofdifferencing circuit 16. Each of the differencing circuits 14-16generates both the sign and the magnitude of the differences betweentheir input signals. The three signals representing the signs of thedifferences are applied to the inputs of override logic block 17. Thethree signals representing the magnitudes of the differences are appliedto the inputs of the keep smallest value block 18 which propagates onlythe smallest of the three values at its input. A multiplexor 19 selectseither the output of the keep smallest value block or zero, depending onthe output of override logic block 17. The value at the output ofmultiplexor 19 is forced to zero if the signs at the outputs ofdifferencing circuits 14 and 15 are the same, of if the signs at theoutputs of differencing circuits 15 and 16 are the same. The value atthe output of multiplexor 19 provides a measure of the motion in thevicinity of pixels P1-P4 according to one aspect of the presentinvention. The local motion value may be integrated over a completefield in order to provide an overall measure of the motion between twofields for the purpose of determining whether the input sequence derivesfrom a film source. Alternatively, the local motion value may be used toadvantage without subsequent integration for the conversion frominterlaced to progressive format of material that has not been derivedfrom film.

In order to fully determine the motion sequence, it is necessary tomeasure a new motion value for each and every field that is received. Inhalf of the cases, the most recent of the two fields is even, while inthe other half the most recent field is odd. In all of the prior artmethods described above, the spatio-temporal relationship of thecontributing pixels relative to one another is fixed irrespective ofwhether the most recent field is even or odd. In a further aspect of thepresent invention, the spatio-temporal relationship is chosen dependingon whether the most recent field is even or odd, so as to generate ameasure of the motion that does not change unduly from one field to thenext. Referring now to FIG. 9, an example of the present invention isprovided which is similar to that shown in FIG. 5, except the imagedetail includes a vertical frequency component that is greater than halfthe vertical frame Nyquist frequency. Note that in this example, themost recent of the two fields is even. Application of the method in thiscase results in a measured motion value of zero, since the value of P3clearly lies between that of P2 and P4. It should be noted that theinventive method produces a value of zero even though in this case theimage detail contains a frequency component outside of the range wherefalse detection is guaranteed to be avoided. This is coincidental andmay occur depending on the phase of the image signal with respect to thesample points.

Referring to FIG. 10, an example is set forth in which the method isapplied to the same image detail set forth in FIG. 9 but where the mostrecent field is odd. In this example, the spatio-temporal relationshipbetween pixels P1 to P4 has been maintained, as in the prior art methodsdescribed earlier. Due to the half line offset between the odd and evenfields, the four contributing pixels have moved along the contour of thestatic image detail, relative to FIG. 9. Application of the method inthis case results in a measured motion value equal to quantity C, sincethe value of P2 does not lie between that of P1 and P3, nor does thevalue of P3 lie between that of P2 and P4. Thus, it can be seen that themeasured motion value may alternate from one field to the next dependingon whether the most recent field is even or odd, despite the fact theremay be no actual motion at all within the image. The inventor hasrealized that this is a detrimental result since alternating high andlow motion values is exactly the same pattern that would be produced byan actual motion sequence produced in accordance with a 2:2 pull downratio, thereby hampering the ability to distinguish motion from staticimages in accordance with the present invention. Consequently, theinventor has concluded that the spatio-temporal relationship between thecontributing pixels should preferably be chosen depending on whether themost recent field is even or odd, as shown in FIG. 11. Essentially, thepixels are chosen such that for a static image, the same image samplesare always used. Thus, if PI represents a sample from an odd field, thenP1 is always taken from an odd field, regardless of whether the mostrecent field is odd or even.

Referring now to FIG. 12, an example is provided of the preferred methodfor choosing the spatio-temporal relationship between the contributingpixels as applied to the example of FIGS. 9 and 10 for the case wherethe most recent field is odd. Application of the formula according tothe method of the present invention yields a measured motion value ofzero, which is the same result as in FIG. 9 where the most recent fieldis even. Thus, undue modulation of the motion value from field to fieldis effectively avoided. It should be noted that in the examples of FIGS.9, 11 and 12, pixel P1 has consistently been taken from the odd field.It will be apparent to one of ordinary skill in the art that pixel P1could have consistently been taken from the even field instead, withresults equal in overall performance.

FIG. 13 shows an apparatus for implementing the method of the presentinvention as shown in FIGS. 9, 11 and 12. For convenience, the samenumbers have been used to designate those items that are in common withthe apparatus shown in FIG. 8. The refinement of appropriately selectingthe pixels so as to avoid modulation of the motion signal from one fieldto the next is achieved by the addition of four multiplexors 20-23 andthrough manipulation of the delay provided by the memory controller 10.It will be apparent from inspection of FIGS. 10 and 12 that the lessdesirable spatio-temporal relationship between the contributing pixelsfor the case in which the most recent field is odd as shown in FIG. 10,can be transformed to the more desirable case as show in FIG. 12, bydelaying the even field by one less line and by subsequentlyinterchanging pixel P1 with P2 and pixel P3 with P4. In the apparatus ofFIG. 13, multiplexors 20 and 21 are used to interchange pixels P3 andP4, while multiplexors 22 and 23 are used to interchange pixels P1 andP2, for the case when the field that is currently being inputted is odd.

A person understanding the present invention may conceive of otherembodiments and variations thereof without departing from the sphere andscope of the invention as defined by the claims appended hereto.

1. A method for measuring motion at a horizontal and vertical positionbetween video fields of opposite parity of a video signal comprising thesteps of: measuring the video signal values of at least two verticallyadjacent pixels from a video field of one parity and at least twovertically adjacent pixels from a video field of the opposite paritysuch that when taken together, the pixels represent contiguous samplesof an image at said horizontal and vertical position; and determining,using differencing circuitry, whether the signal value of any of saidpixels lies between the signal values of adjacent pixels in the field ofopposite parity and in response outputting a zero motion value,otherwise, outputting a motion value equal to the lowest absolutedifference between any of said pixels and its closest adjacent pixel inthe field of opposite parity; and converting the video signal frominterlaced to progressive format using the motion value.
 2. The methodof claim 1 wherein said pixels are measured from the same verticalpositions in fields of like parity, irrespective of the order in whichthe fields were received.
 3. The method of claim 1 wherein twovertically adjacent pixels are taken from an even video field and twovertically adjacent pixels are taken from an odd video field.
 4. Themethod of claim 1 wherein motion values produced from each of aplurality of sets of vertically adjacent pixels are summed substantiallyover an entire field to produce an overall measure of the motion betweensaid fields of opposite parity.
 5. Apparatus for measuring motion at ahorizontal and vertical position between video fields of opposite paritycomprising: register means for selecting at least two verticallyadjacent pixels from a video field of one parity and at least twovertically adjacent pixels from a video field of the opposite paritysuch that when taken together, the pixels represent contiguous samplesof an image at said horizontal and vertical position; and differencingcircuitry for determining whether the signal value of any of said pixelslies between the signal values of adjacent pixels in the field ofopposite parity and in response outputting a zero motion value,otherwise, outputting a motion value equal to the lowest absolutedifference between any of said pixels and its closest adjacent pixel inthe field of opposite parity.
 6. The apparatus of claim 5 wherein saidpixels are measured from the same vertical positions in fields of likeparity, irrespective of the order in which the fields were received. 7.The apparatus of claim 5 wherein two vertically adjacent pixels aretaken from an even video field and two vertically adjacent pixels aretaken from an odd video field.
 8. The apparatus of claim 5 whereinmotion values produced from each of a plurality of sets of verticallyadjacent pixels are summed substantially over an entire field to producean overall measure of the motion between said fields of opposite parity.9. Apparatus for measuring motion at a horizontal and vertical positionbetween video fields of opposite parity comprising: means for measuringthe signal values of at least two vertically adjacent pixels from avideo field of one parity and at least two vertically adjacent pixelsfrom a video field of the opposite parity such that when taken together,the pixels represent contiguous samples of an image at said horizontaland vertical position; and differencing circuitry for determiningwhether the signal value of any of said pixels lies between the signalvalues of adjacent pixels in the field of opposite parity and inresponse outputting a zero motion value, otherwise, outputting a motionvalue equal to the lowest absolute difference between any of said pixelsand its closest adjacent pixel in the field of opposite parity.
 10. Theapparatus of claim 9 wherein said pixels are measured from the samevertical positions in fields of like parity, irrespective of the orderin which the fields were received.
 11. The apparatus of claim 9 whereintwo vertically adjacent pixels are taken from an even video field andtwo vertically adjacent pixels are taken from an odd video field. 12.The apparatus of claim 9 wherein motion values produced from each of aplurality of sets of vertically adjacent pixels are summed substantiallyover an entire field to produce an overall measure of the motion betweensaid fields of opposite parity.